sd-boot: introduce and use efivar_unset()

sd-boot: introduce and use efivar_unset()

Currently some of the code base check for the variable presence before
removing it, and some do not.

More so, in all cases (being updated) we're dealing with non-volatile
variables where changing those attribute to NVRAM wear out.

From what information I could find, there is no definitive answer if the
UEFI implementation will write to the NVRAM even when the variable is
missing.

So add a simple helper that checks for the variable presence before
removing it. While also having a bit cleaner API than the current
efivar_set(..., NULL, ...);

efivar_unset() follows the design from efivar_set*() where it returns an
EFI_STATUS even though its (presently) unused.

v2:
 - add inline comment, use early return

v3:
 - typos? typos!

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>